The present disclosure is directed to aircraft structural assemblies of manned or unmanned airborne vehicles, and more particularly to aircraft structural assemblies for providing precipitation static and lightning strike protection.
The principal material used in the construction of aircraft for much of the past half-century has been aluminum. Aluminum, an excellent conductor, is a highly electrically conductive material and therefore is ideal for dissipating electrical energy across the surface of the aircraft. This conductivity is particularly useful in case of lightning strikes where a large amount of current is transferred to the aircraft over a short period of time.
Electrical energy may also build up on the aircraft due to precipitation static (“P-Static”) resulting from friction between the aircraft and airborne particles, particularly snow, rain, dust, or ice. In these cases, it is important to transmit the electrical energy to static dischargers to reduce interference with instruments and communications in the aircraft.
Aircraft that have an exterior skin that is formed substantially of aluminum have few problems dissipating energy from P-Static and for locations where it is especially high, use static dischargers to control static build-up. The low electrical resistance of aluminum makes distribution of electrical energy a low concern. However, the higher electrical resistance of carbon fiber, or the much higher electrical resistance of dielectric fibers such as glass or aramid, makes interference due to P-Static more likely. Both of the above-described solutions also help mitigate this problem.
One method to increase the conductivity of the composite outer skin of an aircraft is to use expanded foil, such as copper, that is placed adjacent to the carbon reinforced substrate to decrease the chance of damage from lightning strikes. This method requires that expanded foil be galvanically compatible with the carbon substrate, or a dielectric substrate (such as a glass epoxy) must be placed between the foil and carbon substrate to provide protection. The latter structure requires additional components electrically coupling the foil to an electrical ground. Splices or seams in the foil have increased resistance which may disrupt the relaxation of charge leading to precipitation static build-up and more localized lightning damage.
With this copper foil solution, which is moderate to highly galvanically compatible with the carbon substrate, the copper tends to corrode especially in the presence of microcracks that propagate from the copper into the substrate as well as up through the paint where they become visible cracks. These visible cracks can then make it easier for the green corrosion products to discolor the paint. The corrosion and microcracking increase maintenance costs and degrade performance. The use of a dielectric adhesive or surfacer to reduce this issue, increases the weight of the aircraft, reducing the effectiveness obtained by utilizing lighter carbon-fiber substrate. This solution is described extensively in, e.g., U.S. Pat. No. 5,417,385 to Arnold et al., the contents of which are hereby incorporated in their entirety.
As material science has advanced, stronger and more lightweight materials have been developed, such as carbon-reinforced plastic and other carbon fiber materials. Use of these materials, either selectively or throughout the aircraft, increases the load capacity of aircraft by reducing structural weight and increasing strength of the materials. However, the low conductivity of carbon reinforced plastic makes it particularly susceptible to damage due to lightning strikes especially when it is painted. In order to reduce the chance of damage due to lightning strikes, several solutions are used to increase conductivity of the carbon reinforced plastic substrate of the aircraft.
Another method of increasing conductivity of the outer skin of the aircraft is the use of P-Static finish consisting of a specialty primer or paint. The P-Static finish provides electrical conductivity to dissipate the precipitation static. However, this finish is highly volatile, not environmentally friendly, and requires restrictive processing, making commercial application time sensitive and difficult. The protection degrades with time. The black color of the most commonly used exterior P-static paint also presents aesthetic issues. While this solution prevents the buildup of precipitation static, it typically does not provide sufficient conductivity to mitigate lightning strikes.
Therefore, there is a need in the art for an improved method and apparatus for reducing damage from lightning strikes and reducing the effects of P-Static on aircraft.